1. Field of the Invention
The present invention relates to methods and apparatus for controlling fluid flow through flow paths. More specifically, the present invention relates to microfluidic methods and circuit devices in which flow path configurations are designed to control pressure drops on selected flow path segments to balance fluid flow among multiple flow paths, leading to efficient operation of the wetted microfluidic circuit.
2. Description of Related Art
Controlling the movement of fluids through channels on a micro-scale has important applications in a number of technologies. For example, in the field of molecular biology and diagnostic testing and detection, polymerase chain reactions (PCR) have been performed in a chip containing microfabricated flow channels (U.S. Pat. Nos. 5,498,392, 5,587,128, and 5,726,026). In the electronics field, thermal ink jet printers use print heads with flow paths through which ink must flow in a well-controlled manner (U.S. Pat. No. 5,119,116). Proper control of fluids through flow paths has been a challenge, because microdimensions impart characteristics and behaviors that are not found in larger scale systems, which are due primarily to the greater influence of surface effects.
The term xe2x80x9csurface effectsxe2x80x9d is used to describe specific characteristics of a surface on a micro-scale. Materials often have unbound electrons, exposed polar molecules, or other molecular level features that generate a surface charge or reactivity characteristic. Due to scaling, these surface effects or surface forces are substantially more pronounced in microstructures than they are in traditionally sized devices. This is particularly true in micro-scale fluid handling systems where the dynamics of fluid movement are governed by external pressures and by attractions and repulsions between liquids and the materials of the microfluidic systems through which they flow.
It is frequently the case that micro-scale fluid handling systems are designed to perform multiple fluid handling steps in parallel, and it is often considered desirable to process fluids in multiple parallel flow paths simultaneously. However, such systems frequently suffer from uneven and irregular fluid flow. Many such problems are due to surface effects such as those mentioned above. Some micro-scale fluid systems fill unevenly. In others, channels fill at different rates. Additionally, some fluid circuits that split samples into multiple reaction chambers may do so unevenly. Those combining samples from multiple reaction chambers may do so incompletely or unevenly.
Such problems may result in incomplete assays or assays conducted with insufficient amounts of reagent or sample. Some of these problems may result in differences in the reaction times for the different assays, thus changing the results. These and other problems may affect the accuracy of assays and the usability of the micro-scale fluid handling systems themselves. Furthermore, uneven filling tends to result in the waste of valuable reagent or sample material, because larger volumes of fluid may be required to insure that all portions of the system are filled completely.
Yet further, many known chip designs have several wells. In order to provide the most compact arrangement of wells and flow paths, the flow paths must often be asymmetrical in design. The flow paths may thus provide different resistances to the flow of fluids filling the wells or draining from the wells. The presence of differential resistance to flow contributes to the unevenness with which the wells are filled and emptied, and therefore further reduces the accuracy of the assay.
Still further, many microfluidic circuits have well designs in which, due to the configuration of the well, fluids tend to stagnate within the well rather than exiting upon entry of a different fluid. Hence, samples or reagents within the well may not be washed or flushed properly at lower pressures. Additionally, gas bubbles, which may skew the results of the assay, may not be effectively removed from the wells. The use of higher volumes may improve washing but results in waste or inefficient use of limited sample and increased reagent costs.
Accordingly, a need becomes apparent for microfluidic circuits in which fluid flow may be regulated. Fluid flow should preferably be well-controlled during both the initial filling of the unwetted circuit and during subsequent introduction of additional reagents or wash solutions to the wetted circuit, so that gas removal and liquid exchange or flushing can be effectively carried out. Preferably, such regulation can be performed with flow paths and wells that are laid out in a compact, and possibly asymmetrical, fashion. Furthermore, a need exists for fluid circuits, and associated well structures, that can be reliably flushed to remove a liquid or gas at a low pressure and with a low volume of fluid. Such fluid circuits and methods for their use are disclosed herein.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available micro-scale fluid handling systems, also called microfluidics systems.
Thus, the present invention discloses a system and method for controlling the flow of fluids through microchannels in multiple parallel flow paths in such a manner that fluid can be evenly distributed among several parallel channels for multiprocessing. The parallel flow paths are configured to have uniform resistances to fluid flow so that fluid is induced to flow through all of the flow paths at substantially the same flow rate. One or more segments of each flow path may be specially configured to provide a desired relative pressure drop to, so that the pressure gradients can be equalized even though the flow paths may not be symmetrical or coextensive to each other.
According to certain embodiments, the flow of the fluid front through the flow paths is initially controlled by structures that act as passive fluid flow barriers, which in the present invention are abrupt changes in the geometry or surface properties of certain portions of the flow paths. These passive fluid flow barriers act to stop fluid flow by creating a passive pressure barrier that may be overcome by sufficient pressure, or by wetting both sides of the barrier.
Unlike flow barriers that require moving parts, the passive fluid flow barriers or abrupt flow path widenings can be static and their operation does not depend upon the use of moving parts. They are thus cheaper and simpler to construct than the various types of microelectromechanical active valves, and they do not require external controls.
According to certain embodiments, a microfluidic circuit within the scope of the invention may have a plurality of flow paths branching from a common inlet. Each flow path may have a filling portion that supplies fluid to an associated well or structure, and a draining portion that receives fluid from the well or structure for collection, further processing, or disposal. Throughout the specification, any such structures, which serve as sites at which chemical reactions and/or read-outs take place, will simply be referred to as wells. However, it should be understood that a well is just an example of a fluid handling structure that may be included in the flow path, and the flow path could include other fluid handling structures, e.g., a combination of several wells in series or parallel, channels or chambers of various dimensions, or chambers containing a matrix material, such as a filter, separation, or binding medium, including fibers, resins, beads or other materials. Any such structures may be used in the practice of the present invention, providing it is possible to identify the contribution of the fluid processing structure to the resistance to fluid flow and pressure drop over that portion of the flow path.
The common inlet may feed a main distribution channel. The wells may be disposed on one or both sides of a main distribution channel that incorporates a portion of at least some of the flow paths. The wells may be disposed in a row. Entrance channels may branch off the main distribution channel at intervals to deliver fluid to wells. The filling portion of each flow path is then made up of the entrance channel and the portion of the main distribution channel leading from the common inlet to the entrance channel. The wells or structures may be arranged in a compact manner to increase the number of wells or structures on a single chip.
Each of the wells or structures may be separated from the draining portion of the associated flow path by a passive fluid flow barrier. More specifically, a plurality of exit channels may intersect each of the wells. The exit channels may be somewhat narrow so that the juncture of each exit channel with a well is a narrowing with the proper geometry to form a passive pressure barrier. The intersection of multiple exit channels with each well may serve to draw fluid from multiple regions of the well simultaneously, thereby avoiding the formation of recirculating currents or fluid stagnation that may otherwise tend to inefficient washing of fluid, and trapping of air bubbles within the wells.
The exit channels from each well may merge with an extension channel that conveys the fluid to a waste collection channel, and from there to a single system outlet, all of which together make up the draining portion of the flow path. The extension channels on different flow paths may differ from each other so that they do not provide the same pressure gradient or pressure drop. More precisely, the pressure drop of each extension channel may be selected to compensate for the difference in pressure drop between the associated flow path and the remaining flow paths. The extension channels may thus have different lengths, cross sectional areas, surface roughness characteristics, or the like. Hence, the extension channels may be structured in such a manner that all of the flow paths have substantially the flow rate when subjected to the same pressure differential.
Extension channels may intersect the waste channel at different points, so that the length of the waste collection channel included between the extension channel and the common system outlet may differ for the different flow paths. According to one embodiment, the waste collection channel is configured in such a manner that the extension channels have different lengths to provide relative pressure drop compensation. The extension channels may be straight to intersect the waste collection channel at different distances. According to an alternative embodiment, the waste collection channel may be straight, and may run parallel to the distribution of the wells. The extension channels may still be different lengths; each of the longer extension channels may have a serpentine configuration. Varying numbers of bends may be used to vary the lengths of the extension channels.
According to another alternative embodiment, the waste collection channel may again be straight and parallel to the distribution of the wells. The extension channels may all be equal in length, but may have different cross sectional areas. Since the pressure drop across an extension channel is generally inversely proportional to the cross sectional area of the extension channel for a given flow rate, variation of the cross sectional areas may be used to equalize the pressure drops of the flow paths.
In operation, fluid flows from the common inlet, through the filling portion of each flow path and into the associated well, until it is stopped by a passive fluid flow barrier when the well has filled; this occurs until all wells have been filled. When it is desired to flush the fluid from the wells, a second fluid may be injected through the inlet at a pressure high enough to overcome the passive fluid flow barriers. The second fluid then pushes the first fluid out through the exit channels relatively evenly, as a result of the fact that all of the flow paths have substantially the same resistance to fluid flow. Furthermore, the use of multiple exit channels at the outlet of each well contributes to even and efficient washing of the first fluid by the second fluid. Air bubbles may also be flushed out through the exit channels, by virtue of the manner in which the exit channels are positioned.
According to alternative embodiments, the well structures may be further modified to enhance the liquid flushing and/or gas removal characteristics of the microfluidic circuit. For example, air vents may also intersect the well, in addition to the exit channels. The air vents may have a small cross-sectional area by comparison with the exit channels, so that they allow gas, such as air, to pass but create a sufficient pressure barrier to prevent liquid from flowing through the vent. Several such air vents may be distributed about the well to provide relatively complete gas removal.
According to another alternative well structure, flow dividers may be provided within the well. Each flow divider may simply be a wall or surface structure that divides a stream of incoming fluid into two streams. Multiple dividers may be used to provide three or more streams, each of which is directed into a different portion of the well. Hence, stagnant fluid that would otherwise tend to remain in the well may be avoided, and gas or liquid may be more completely flushed from the fluid circuit by the entrance of a second fluid.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.